block includes
  include ../_util-fns
:marked
  This page describes Angular's built-in
  protections against common web-application vulnerabilities and attacks such as cross-site
  scripting attacks. It doesn't cover application-level security, such as authentication (_Who is
  this user?_) and authorization (_What can this user do?_).

  For more information about the attacks and mitigations described below, see [OWASP Guide Project](https://www.owasp.org/index.php/Category:OWASP_Guide_Project).
  
.l-main-section
:marked
  # Contents

  * [Reporting vulnerabilities](#report-issues).
  * [Best practices](#best-practices).
  * [Preventing cross-site scripting (XSS)](#xss).
  * [Trusting safe values](#bypass-security-apis).
  * [HTTP-Level vulnerabilities](#http).
  * [Auditing Angular applications](#code-review).

:marked
  You can run the <live-example></live-example> in Plunker and download the code from there.

.l-main-section
h2#report-issues Reporting vulnerabilities
:marked
  To report vulnerabilities in Angular itself, email us at [security@angular.io](mailto:security@angular.io).

  For more information about how Google handles security issues, see [Google's security
  philosophy](https://www.google.com/about/appsecurity/).

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h2#best-practices Best practices
:marked
  * **Keep current with the latest Angular library releases.**
  We regularly update the Angular libraries, and these updates may fix security defects discovered in
  previous versions. Check the Angular [change
  log](https://github.com/angular/angular/blob/master/CHANGELOG.md) for security-related updates.

  * **Don't modify your copy of Angular.**
  Private, customized versions of Angular tend to fall behind the current version and may not include
  important security fixes and enhancements. Instead, share your Angular improvements with the
  community and make a pull request.

  * **Avoid Angular APIs marked in the documentation as “_Security Risk_.”**
  For more information, see the [Trusting safe values](#bypass-security-apis) section of this page.

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h2#xss Preventing cross-site scripting (XSS)
:marked
  [Cross-site scripting (XSS)](https://en.wikipedia.org/wiki/Cross-site_scripting) enables attackers
  to inject malicious code into web pages. Such code can then, for example, steal user data (in
  particular, login data) or perform actions to impersonate the user. This is one of the most
  common attacks on the web.

  To block XSS attacks, you must prevent malicious code from entering the DOM (Document Object Model). For example, if
  attackers can trick you into inserting a `<script>` tag in the DOM, they can run arbitrary code on
  your website. The attack isn't limited to `<script>` tags&mdash;many elements and properties in the
  DOM allow code execution, for example, `<img onerror="...">` and `<a href="javascript:...">`. If
  attacker-controlled data enters the DOM, expect security vulnerabilities.

  ### Angular’s cross-site scripting security model

  To systematically block XSS bugs, Angular treats all values as untrusted by default. When a value
  is inserted into the DOM from a template, via property, attribute, style, class binding, or interpolation, 
  Angular sanitizes and escapes untrusted values.

  _Angular templates are the same as executable code_: HTML, attributes, and binding expressions
  (but not the values bound) in templates are trusted to be safe. This means that applications must
  prevent values that an attacker can control from ever making it into the source code of a
  template. Never generate template source code by concatenating user input and templates. 
  To prevent these vulnerabilities, use
  the [offline template compiler](#offline-template-compiler), also known as _template injection_.

  ### Sanitization and security contexts

  _Sanitization_ is the inspection of an untrusted value, turning it into a value that's safe to insert into
  the DOM. In many cases, sanitization doesn't change a value at all. Sanitization depends on context:
  a value that's harmless in CSS is potentially dangerous in a URL.

  Angular defines the following security contexts:

  * **HTML** is used when interpreting a value as HTML, for example, when binding to `innerHtml`.
  * **Style** is used when binding CSS into the `style` property.
  * **URL** is used for URL properties, such as `<a href>`.
  * **Resource URL** is a URL that will be loaded and executed as code, for example, in `<script src>`.

  Angular sanitizes untrusted values for HTML, styles, and URLs; sanitizing resource URLs isn't
  possible because they contain arbitrary code. In development mode, Angular prints a console warning
  when it has to change a value during sanitization.

  ### Sanitization example

  The following template binds the value of `htmlSnippet`, once by interpolating it into an element's
  content, and once by binding it to the `innerHTML` property of an element:

+makeExample('src/app/inner-html-binding.component.html')

:marked
  Interpolated content is always escaped&mdash;the HTML isn't interpreted and the browser displays
  angle brackets in the element's text content.

  For the HTML to be interpreted, bind it to an HTML property such as `innerHTML`. But binding
  a value that an attacker might control into `innerHTML` normally causes an XSS
  vulnerability. For example, code contained in a `<script>` tag is executed:

+makeExcerpt('src/app/inner-html-binding.component.ts', 'class')

:marked
  Angular recognizes the value as unsafe and automatically sanitizes it, which removes the `<script>`
  tag but keeps safe content such as the text content of the `<script>` tag and the `<b>` element.

figure.image-display
    img(src='/resources/images/devguide/security/binding-inner-html.png'
        alt='A screenshot showing interpolated and bound HTML values')

:marked
  ### Avoid direct use of the DOM APIs

  The built-in browser DOM APIs don't automatically protect you from security vulnerabilities.
  For example, `document`, the node available through `ElementRef`, and many third-party APIs
  contain unsafe methods. Avoid directly interacting with the DOM and instead use Angular
  templates where possible.

  ### Content security policy

  Content Security Policy (CSP) is a defense-in-depth
  technique to prevent XSS. To enable CSP, configure your web server to return an appropriate
  `Content-Security-Policy` HTTP header. Read more about content security policy at 
  [An Introduction to Content Security Policy](http://www.html5rocks.com/en/tutorials/security/content-security-policy/)
  on the HTML5Rocks website.

a#offline-template-compiler
:marked
  ### Use the offline template compiler

  The offline template compiler prevents a whole class of vulnerabilities called template injection,
  and greatly improves application performance. Use the offline template compiler in production
  deployments; don't dynamically generate templates. Angular trusts template code, so generating
  templates, in particular templates containing user data, circumvents Angular's built-in protections. 
  For information about dynamically constructing forms in a safe way, see the 
  [Dynamic Forms](../cookbook/dynamic-form.html) cookbook page.

  ### Server-side XSS protection

  HTML constructed on the server is vulnerable to injection attacks. Injecting template code into an
  Angular application is the same as injecting executable code into the
  application: it gives the attacker full control over the application. To prevent this, 
  use a templating language that automatically escapes values to prevent XSS vulnerabilities on
  the server. Don't generate Angular templates on the server side using a templating language; doing this
  carries a high risk of introducing template-injection vulnerabilities.

.l-main-section
h2#bypass-security-apis Trusting safe values
:marked
  Sometimes applications genuinely need to include executable code, display an `<iframe>` from some
  URL, or construct potentially dangerous URLs. To prevent automatic sanitization in any of these 
  situations, you can tell Angular that you inspected a value, checked how it was generated, and made 
  sure it will always be secure. But *be careful*. If you trust a value that might be malicious, you 
  are introducing a security vulnerability into your application. If in doubt, find a professional 
  security reviewer.

  To mark a value as trusted, inject `DomSanitizer` and call one of the
  following methods:

  * `bypassSecurityTrustHtml`
  * `bypassSecurityTrustScript`
  * `bypassSecurityTrustStyle`
  * `bypassSecurityTrustUrl`
  * `bypassSecurityTrustResourceUrl`

  Remember, whether a value is safe depends on context, so choose the right context for
  your intended use of the value. Imagine that the following template needs to bind a URL to a
  `javascript:alert(...)` call:

+makeExcerpt('src/app/bypass-security.component.html', 'URL')

:marked
  Normally, Angular automatically sanitizes the URL, disables the dangerous code, and
  in development mode, logs this action to the console. To prevent
  this, mark the URL value as a trusted URL using the `bypassSecurityTrustUrl` call:

+makeExcerpt('src/app/bypass-security.component.ts ()', 'trust-url')

figure.image-display
    img(src='/resources/images/devguide/security/bypass-security-component.png'
        alt='A screenshot showing an alert box created from a trusted URL')

:marked
  If you need to convert user input into a trusted value, use a
  controller method. The following template allows users to enter a YouTube video ID and load the
  corresponding video in an `<iframe>`. The `<iframe src>` attribute is a resource URL security
  context, because an untrusted source can, for example, smuggle in file downloads that unsuspecting users 
  could execute. So call a method on the controller to construct a trusted video URL, which causes
  Angular to allow binding into `<iframe src>`:

+makeExcerpt('src/app/bypass-security.component.html', 'iframe')
+makeExcerpt('src/app/bypass-security.component.ts ()', 'trust-video-url')

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h2#http HTTP-level vulnerabilities
:marked
  Angular has built-in support to help prevent two common HTTP vulnerabilities, cross-site request
  forgery (CSRF or XSRF) and cross-site script inclusion (XSSI). Both of these must be mitigated primarily 
  on the server side, but Angular provides helpers to make integration on the client side easier.

h3#xsrf Cross-site request forgery
:marked
  In a cross-site request forgery (CSRF or XSRF), an attacker tricks the user into visiting
  a different web page (such as `evil.com`) with malignant code that secretly sends a malicious request 
  to the application's web server (such as `example-bank.com`). 

  Assume the user is logged into the application at `example-bank.com`.
  The user opens an email and clicks a link to `evil.com`, which opens in a new tab.

  The `evil.com` page immediately sends a malicious request to `example-bank.com`.
  Perhaps it's a request to transfer money from the user's account to the attacker's account.
  The browser automatically sends the `example-bank.com` cookies (including the authentication cookie) with this request.

  If the `example-bank.com` server lacks XSRF protection, it can't tell the difference between a legitimate 
  request from the application and the forged request from `evil.com`.

  To prevent this, the application must ensure that a user request originates from the real
  application, not from a different site. 
  The server and client must cooperate to thwart this attack.

  In a common anti-XSRF technique, the application server sends a randomly
  generated authentication token in a cookie.
  The client code reads the cookie and adds a custom request header with the token in all subsequent requests.
  The server compares the received cookie value to the request header value and rejects the request if the values are missing or don't match.

  This technique is effective because all browsers implement the _same origin policy_. Only code from the website 
  on which cookies are set can read the cookies from that site and set custom headers on requests to that site.
  That means only your application can read this cookie token and set the custom header. The malicious code on `evil.com` can't.

  Angular's `http` has built-in support for the client-side half of this technique in its `XSRFStrategy`. 
  The default `CookieXSRFStrategy` is turned on automatically.
  Before sending an HTTP request, the `CookieXSRFStrategy` looks for a cookie called `XSRF-TOKEN` and 
  sets a header named `X-XSRF-TOKEN` with the value of that cookie. 

  The server must do its part by setting the
  initial `XSRF-TOKEN` cookie and confirming that each subsequent state-modifying request
  includes a matching `XSRF-TOKEN` cookie and `X-XSRF-TOKEN` header.

  XSRF/CSRF tokens should be unique per user and session, have a large random value generated by a
  cryptographically secure random number generator, and expire in a day or two.

  Your server may use a different cookie or header name for this purpose.
  An Angular application can customize cookie and header names by providing its own `CookieXSRFStrategy` values.
code-example(language="typescript").
  { provide: XSRFStrategy, useValue: new CookieXSRFStrategy('myCookieName', 'My-Header-Name') }
:marked
  Or you can implement and provide an entirely custom `XSRFStrategy`:

code-example(language="typescript").
  { provide: XSRFStrategy, useClass: MyXSRFStrategy }

:marked
  For information about CSRF at the Open Web Application Security Project (OWASP), see
  <a href="https://www.owasp.org/index.php/Cross-Site_Request_Forgery_%28CSRF%29" target="_blank">Cross-Site Request Forgery (CSRF)</a> and
  <a href="https://www.owasp.org/index.php/CSRF_Prevention_Cheat_Sheet" target="_blank">Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet</a>.
  The Stanford University paper 
  <a href="https://seclab.stanford.edu/websec/csrf/csrf.pdf" target="_blank">Robust Defenses for Cross-Site Request Forgery</a> is a rich source of detail.

  See also Dave Smith's easy-to-understand 
  <a href="https://www.youtube.com/watch?v=9inczw6qtpY" target="_blank" title="Cross Site Request Funkery Securing Your Angular Apps From Evil Doers">talk on XSRF at AngularConnect 2016</a>.

h3#xssi Cross-site script inclusion (XSSI)
:marked
  Cross-site script inclusion, also known as JSON vulnerability, can allow an attacker's website to
  read data from a JSON API. The attack works on older browsers by overriding native JavaScript
  object constructors, and then including an API URL using a `<script>` tag.

  This attack is only successful if the returned JSON is executable as JavaScript. Servers can
  prevent an attack by prefixing all JSON responses to make them non-executable, by convention, using the
  well-known string `")]}',\n"`.

  Angular's `Http` library recognizes this convention and automatically strips the string
  `")]}',\n"` from all responses before further parsing.

  For more information, see the XSSI section of this [Google web security blog
  post](https://security.googleblog.com/2011/05/website-security-for-webmasters.html).

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h2#code-review Auditing Angular applications
:marked
  Angular applications must follow the same security principles as regular web applications, and
  must be audited as such. Angular-specific APIs that should be audited in a security review,
  such as the [_bypassSecurityTrust_](#bypass-security-apis) methods, are marked in the documentation
  as security sensitive.
